1. Field of The Invention
The invention relates generally to temperature detectors used in conjunction with electrical connectors, and more particularly to insulation displacement type electrical connectors utilized in outdoor lighting systems.
2. Description of Related Technology
Low voltage outdoor lighting systems typically include several components. Among these components are an electrical transformer for converting standard household (120 volts) alternating current into a lower voltage (approximately 12 volts) alternating current. Such systems also utilize various lighting fixtures which are typically mounted above the ground. These fixtures include sockets for low voltage electrical lamps and lenses for refracting light emitted by the lamp. The light fixtures are typically mounted or placed in the ground some distance from the power transformer which is typically located or mounted on the side of the home. The lighting fixtures are connected to, and supplied with electric current from the transformer by a series of main and possibly smaller feeder electrical cables that run underground from the transformer location to the various light fixture locations. In some cases, the light fixtures are connected, physically and electrically, directly to the main cable without any feeder cable running between the lighting fixture and the main cable. In those cases, the light fixture has means incorporated in it for directly connecting to the main cable.
Typically, one main cable runs from the transformer to the ground area where the lighting fixtures are placed. This main cable is relatively large as it must carry the total current for the sum of the lighting fixtures. The cable must be extended or a branch line added on occasion as required by the particular installation, which requires that an additional length of the main cable be connected to the original length of main cable by a connecting device or terminal, which physically and electrically joins the two cables to each other. This interconnection of the main cables is accomplished by a cable connector. Since the installation is to be accomplished in the field by a person not skilled in electrical work, the connector is preferably of the insulation displacement type, thereby eliminating the use of solder, special tools, or a knowledge of standard cable splicing techniques.
An example of such previously employed connectors is disclosed in U.S. Pat. No. 5,007,855, which includes a plurality of electrically conductive jumper elements, wherein each of the jumper elements is formed to include a pair of sharp protrusions. The cable connector also includes a cap wherein the cap consists of an electrically insulating material which engages the electrically conductive jumper elements. The cable connector, commonly known as an insulation displacement type cable connector, also includes a base formed of an electrically insulating material so as to have a plurality of channel like cavities for accepting the electrical cables. The base nests within the cap and the cavities within the base are positioned so as to receive the protrusions of the conductive jumper elements engaged by the cap. A self tapping screw fastens the cap and base together as the protrusions penetrate the insulating jacket surrounding the cable and thereby make electrical contact with the cores of the cables that are being interconnected. Current can then be conducted between the two cables.
In addition, the state of the art for such connectors is exemplified by U.S. Pat. No. 5,055,071 (Carlson) discloses an in line fuse holder, and by U.S. Pat. No. 4,337,374 (Smith), which discloses a service wire splice enclosure. The main feature of the device is that it provides a moisture proof enclosure for cable splices.
In the insulation displacement type of connector such as those discussed above, the quality of the electrical connection can vary over time based on the quality of installation as well as environmental factors such as heating and cooling cycles, moisture, debris and corrosion. One indication that the electrical connection is poor in a given connector is the generation of increasing amounts of heat at the high resistance portions of the deteriorating junction. Corrosion and heat can lead to arcing, current flow through a diminished cross sectional area and thermal damage to the connector itself.
Therefore, sensing of the heat generated at a poor electrical connection and removing electrical current from the circuit after the temperature of the electrical wiring exceeds a predetermined threshold is desirable. U.S. Pat. No. 5,177,568 (Brenner et. al.) discloses a heat flow detector which is used in conjunction with a junction box which supplies electricity to an electric lamp. The Brenner disclosure is directed toward the problem of excess heat generated by a combination of improperly sized lights and excess amounts of insulation. Brenner utilizes an internal bimetallic switch that opens the circuit when the temperature exceeds a certain point, but when the connector has cooled, the bimetallic switch automatically closes. When a lamp is connected to such a connector, overheating will cause the lamp to cycle on and off indefinitely. In order for the bimetallic switch to operate effectively, an electrical resistor is positioned adjacent to the connector for the purpose of elevating the temperature of the connector to a predetermined threshold temperature. The threshold temperature is chosen so that spurious normal temperature variations are ignored by the bimetallic switch.
Another circuit interrupter that is heat sensitive is disclosed in U.S. Pat. No. 4,581,674 (Brzozowski). The Brzozowski disclosure is directed towards the terminal connection at which house wiring is attached to wall sockets. If the connection at the terminals is inadequate due to excessive resistance between the house wiring and the connectors, a subsequent electrical load (a lamp for example) will cause the junction to generate heat. Unfortunately, the circuit is not completely broken in response to an overheated condition, but rather the current is rerouted through a neon lamp which serves as an indicating device. Thus, current is still being supplied to the load and heat is still being generated at the degraded terminal connection. Further, since neon lamps tend to fail explosively, another hazard is created when additional current is rerouted through the lamp after the heat sensitive fuse opens.
U.S. Pat. No. 4,776,809 (Hall) discloses a low voltage distribution system for lamps which utilizes bimetallic strip to protect the circuit in response to excessive current flow.
U.S. Pat. No. 4,513,238 (Orban) discloses an automatic battery charger with a thermal control. In the Orban device, the heat to which a thermal switch responds is provided entirely by resistors rather than a combination of resistors and latent heat build up. The thermal switch is used to switch the battery charging circuit from a fast to a slow charging rate. Once the switch causes the slow charging circuit to actuate, it stays in the slow charge mode until the charger is turned off.
U.S. Pat. No. 4,203,053 (Shepard) discloses a low voltage distribution system for a miniature structure such as a doll house. Shepard discloses an adhesively backed, electrically supplied foil strip and a plurality of bipronged electrical fastening members which are removably attached thereto. The fastening members are preferably miniature light bulbs. A transformer is used to provide reduced voltage to the foil strip. If the transformer becomes overloaded due to high current flow, a thermal cutout may be used to open the circuit.
U.S. Pat. No. 2,920,163 (Johnson) discloses an electrical connector for building panels. The connector utilizes a fuse element which is actuated when current above a predetermined value passes through it, as would be the case when a short circuit or overload occurs.
U.S. Pat. No. 2,704,797 (Fettwies) discloses a thermally activated switch for use in a fire alarm. The device utilizes a bakelite housing in which the switch is contained, and a cap which holds a fuse pin. The fuse pin is made of a material which has a low melting point. A portion of the fuse pin is positioned so that it biases an elastic contact strip away from a tongue. When the fuse melts in response to high temperature, the elastic contact strip contacts the tongue and completes a circuit. The cap is fabricated of a heat conducting material and serves as a heat sink to permit sensing of external temperatures.
U.S. Pat. No. 2,204,237 (Slack et. al.) discloses a cable alarm and switch. The switch can be inserted between flexible connectors in a conventional electrical wire. The switch is provided with a spring biased contact which is retained in an open position by a fusible band of low melting point material. When the external temperature reaches a predetermined level, the fusible material melts and the contact completes the circuit.
U.S. Pat. No. 1,539,293 (Bridges) discloses a combined thermostatic circuit closer and sprinkler. The sprinkler is activated in response to high ambient temperature. The fusible material is used to bias a spring loaded valve in a closed position. When exposed to high temperatures, the fuse material melts and the valve is allowed to spring open. The resulting stream of water pushes against a rotatable spring loaded wheel and causes a flange to contact studs which are in electrical communication with the wires of an electrical circuit. When the flange contacts both of the studs, the circuit is completed and an alarm is sounded.
U.S. Pat. No. 1,156,717 (Smith) discloses an automatic fire alarm. Smith discloses an electrical circuit which is completed when a weight falls into a tube, thereby causing the physical interconnection of two contacts. The weight is normally held in an inactive position by a meltable material. In response to high ambient temperature, the weight is allowed to fall and thus complete the circuit.
U.S. Pat. No. 771,144 (Gould) discloses a thermostat which includes a coaxial cable to which is added a fusible material placed between the conducting elements. When the ambient temperature reaches a predetermined level, the fusible material melts, thereby bridging the coaxial elements and completing a circuit.